good for new students

2. BRAIN RULES. Copyright © 2008 by John J. Medina.
All rights reserved. Printed in the United States of America.
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First Pear Press trade paperback edition 2009
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ISBN-10: 0-9797777-4-7
ISBN-13: 978-0-9797777-4-5
10 9 8 7 6 5 4 3 2 1

3. To Joshua and Noah
Gratitude, my dear boys, for constantly reminding me
that age is not something that matters unless you are cheese.

5. contents
introduction 1
exercise 7
Rule #1: Exercise boosts brain power.
Our brains love motion ~ The incredible test-score booster ~ Will you age like
Jim or like Frank? ~ How oxygen builds roads for the brain
survival 29
Rule #2:The human brain evolved, too.
What’s uniquely human about us ~ A brilliant survival strategy ~ Meet your
brain ~ How we conquered the world
wiring 49
Rule #3: Every brain is wired differently.
Neurons slide, slither, and split ~ Experience makes the difference ~ Furious
brain development not once, but twice ~ The Jennifer Aniston neuron
attention 71
Rule #4:We don’t pay attention to boring things.
Emotion matters ~ Why there is no such thing as multitasking ~ We pay great
attention to threats, sex, and pattern matching ~ The brain needs a break!
short-term memory 95
Rule #5: Repeat to remember.
Memories are volatile ~ How details become splattered across the insides of our
brains ~ How the brain pieces them back together again ~ Where memories go
long-term memory 121
Rule #6: Remember to repeat.
If you don’t repeat this within 30 seconds, you’ll forget it ~ Spaced repetition
cycles are key to remembering ~ When floating in water could help your memory

6. sleep 149
Rule #7: Sleep well, think well.
The brain doesn’t sleep to rest ~ Two armies at war in your head ~ How to
improve your performance 34 percent in 26 minutes ~ Which bird are you? ~
Sleep on it!
stress 169
Rule #8: Stressed brains don’t learn the same way.
Stress is good, stress is bad ~ A villain and a hero in the toxic-stress battle ~ Why
the home matters to the workplace ~ Marriage intervention for happy couples
sensory integration 197
Rule #9: Stimulate more of the senses.
Lessons from a nightclub ~ How and why all of our senses work together ~
Multisensory learning means better remembering ~ What’s that smell?
vision 221
Rule #10: Vision trumps all other senses.
Playing tricks on wine tasters ~ You see what your brain wants to see, and it
likes to make stuff up ~ Throw out your PowerPoint
gender 241
Rule #11: Male and female brains are different.
Sexing humans ~ The difference between little girl best friends and little boy best
friends ~ Men favor gist when stressed; women favor details ~ A forgetting drug
exploration 261
Rule #12: We are powerful and natural explorers.
Babies are great scientists ~ Exploration is aggressive ~ Monkey see, monkey do
~ Curiosity is everything
acknowledgements 283
index 285

9. go ahead and multiply the number 8,388,628 x 2 in your head.
Can you do it in a few seconds? There is a young man who can double
that number 24 times in the space of a few seconds. He gets it right
every time. There is a boy who can tell you the precise time of day
at any moment, even in his sleep. There is a girl who can correctly
determine the exact dimensions of an object 20 feet away. There is
a child who at age 6 drew such lifelike and powerful pictures, she
got her own show at a gallery on Madison Avenue. Yet none of these
children could be taught to tie their shoes. Indeed, none of them
have an IQ greater than 50.
The brain is an amazing thing.
Your brain may not be nearly so odd, but it is no less
extraordinary. Easily the most sophisticated information-transfer
system on Earth, your brain is fully capable of taking the little black
squiggles on this piece of bleached wood and deriving meaning from
them. To accomplish this miracle, your brain sends jolts of electricity
crackling through hundreds of miles of wires composed of brain cells
introduction

10. BRAIN RULES
2
so small that thousands of them could fit into the period at the end
of this sentence. You accomplish all of this in less time than it takes
you to blink. Indeed, you have just done it. What’s equally incredible,
given our intimate association with it, is this: Most of us have no idea
how our brain works.
This has strange consequences. We try to talk on our cell phones
and drive at the same time, even though it is literally impossible for
our brains to multitask when it comes to paying attention. We have
created high-stress office environments, even though a stressed brain
is significantly less productive. Our schools are designed so that most
real learning has to occur at home. This would be funny if it weren’t
so harmful. Blame it on the fact that brain scientists rarely have a
conversation with teachers and business professionals, education
majors and accountants, superintendents and CEOs. Unless you have
the Journal of Neuroscience sitting on your coffee table, you’re out of
the loop.
This book is meant to get you into the loop.
12 brain rules
My goal is to introduce you to 12 things we know about how
the brain works. I call these Brain Rules. For each rule, I present
the science and then offer ideas for investigating how the rule
might apply to our daily lives, especially at work and school. The
brain is complex, and I am taking only slivers of information from
each subject—not comprehensive but, I hope, accessible. The Brain
Rules film, available at www.brainrules.net/dvd, is an integral part of
the project. You might use the DVD as an introduction, and then
jump between a chapter in the book and the illustrations online. A
sampling of the ideas you’ll encounter:
• For starters, we are not used to sitting at a desk for eight hours
a day. From an evolutionary perspective, our brains developed
while working out, walking as many as 12 miles a day. The brain
still craves that experience, especially in sedentary populations like

11. INTRODUCTION
3
our own. That’s why exercise boosts brain power (Brain Rule #1) in
such populations. Exercisers outperform couch potatoes in long-
term memory, reasoning, attention, and problem-solving tasks. I am
convinced that integrating exercise into our eight hours at work or
school would only be normal.
• As you no doubt have noticed if you’ve ever sat through a typical
PowerPoint presentation, people don’t pay attention to boring things
(Brain Rule #4). You’ve got seconds to grab someone’s attention and
only 10 minutes to keep it. At 9 minutes and 59 seconds, something
must be done to regain attention and restart the clock—something
emotional and relevant. Also, the brain needs a break. That’s why I
use stories in this book to make many of my points.
• Ever feel tired about 3 o’clock in the afternoon? That’s because
your brain really wants to take a nap. You might be more productive
if you did: In one study, a 26-minute nap improved NASA pilots’
performance by 34 percent. And whether you get enough rest at
night affects your mental agility the next day. Sleep well, think well
(Brain Rule #7).
• We’ll meet a man who can read two pages at the same time, one
with each eye, and remember everything in the pages forever. Most
of us do more forgetting than remembering, of course, and that’s why
we must repeat to remember (Brain Rule #5). When you understand
the brain’s rules for memory, you’ll see why I want to destroy the
notion of homework.
• We’ll find out why the terrible twos only look like active
rebellion but actually are a child’s powerful urge to explore. Babies
may not have a lot of knowledge about the world, but they know a
whole lot about how to get it. We are powerful and natural explorers
(Brain Rule #12), and this never leaves us, despite the artificial
environments we’ve built for ourselves.
no prescriptions
The ideas ending the chapters of this book are not a prescription.

12. BRAIN RULES
4
They are a call for real-world research. The reason springs from
what I do for a living. My research expertise is the molecular basis of
psychiatric disorders, but my real interest is in trying to understand
the fascinating distance between a gene and a behavior. I have been
a private consultant for most of my professional life, a hired gun for
research projects in need of a developmental molecular biologist with
such specialization. I have had the privilege of watching countless
research efforts involving chromosomes and mental function.
On such journeys, I occasionally would run across articles and
books that made startling claims based on “recent advances” in
brain science about how to change the way we teach people and do
business. And I would panic, wondering if the authors were reading
some literature totally off my radar screen. I speak several dialects
of brain science, and I knew nothing from those worlds capable of
dictating best practices for education and business. In truth, if we
ever fully understood how the human brain knew how to pick up a
glass of water, it would represent a major achievement.
There was no need to panic. You can responsibly train a skeptical
eye on any claim that brain research can without equivocation tell us
how to become better teachers, parents, business leaders, or students.
This book is a call for research simply because we don’t know enough
to be prescriptive. It is an attempt to vaccinate against mythologies
such as the “Mozart Effect,” left brain/right brain personalities, and
getting your babies into Harvard by making them listen to language
tapes while they are still in the womb.
back to the jungle
What we know about the brain comes from biologists who
study brain tissues, experimental psychologists who study behavior,
cognitive neuroscientists who study how the first relates to the
second, and evolutionary biologists. Though we know precious little
about how the brain works, our evolutionary history tells us this: The
brain appears to be designed to solve problems related to surviving

13. INTRODUCTION
5
in an unstable outdoor environment, and to do so in nearly constant
motion. I call this the brain’s performance envelope.
Each subject in this book—exercise, survival, wiring, attention,
memory, sleep, stress, sense, vision, gender, and exploration—
relates to this performance envelope. Motion translates to exercise.
Environmental instability led to the extremely flexible way our
brains are wired, allowing us to solve problems through exploration.
Learning from our mistakes so we could survive in the great outdoors
meant paying attention to certain things at the expense of others,
and it meant creating memories in a particular way. Though we have
been stuffing them into classrooms and cubicles for decades, our
brains actually were built to survive in jungles and grasslands. We
have not outgrown this.
I am a nice guy, but I am a grumpy scientist. For a study
to appear in this book, it has to pass what some at The Boeing
Company (for which I have done some consulting) call MGF: the
Medina Grump Factor. That means the supporting research for
each of my points must first be published in a peer-reviewed journal
and then successfully replicated. Many of the studies have been
replicated dozens of times. (To stay as reader-friendly as possible,
extensive references are not in this book but can be found at
www.brainrules.net.)
What do these studies show, viewed as a whole? Mostly this: If
you wanted to create an education environment that was directly
opposed to what the brain was good at doing, you probably would
design something like a classroom. If you wanted to create a business
environment that was directly opposed to what the brain was good
at doing, you probably would design something like a cubicle. And if
you wanted to change things, you might have to tear down both and
start over.
In many ways, starting over is what this book is all about.

15. exercise
Rule #1
Exercise boosts brain power.

17. if the cameras weren’t rolling and the media abuzz
with live reports, it is possible nobody would have
believed the following story:
A man had been handcuffed, shackled and thrown
into California’s Long Beach Harbor, where he was quickly fastened
to a floating cable. The cable had been attached at the other end to
70 boats, bobbing up and down in the harbor, each carrying a single
person. Battling strong winds and currents, the man then swam,
towing all 70 boats (and passengers) behind him, traveling 1.5 miles
to Queen’s Way Bridge. The man, Jack La Lanne, was celebrating his
birthday.
He had just turned 70 years old.
Jack La Lanne, born in 1914, has been called the godfather of
the American fitness movement. He starred in one of the longest-
running exercise programs produced for commercial television. A
prolific inventor, La Lanne designed the first leg-extension machines,
the first cable-fastened pulleys, and the first weight selectors, all now

18. BRAIN RULES
10
standard issue in the modern gym. He is even credited with inventing
an exercise that supposedly bears his name, the Jumping Jack. La
Lanne is now in his mid-90s, and even these feats are probably not
the most interesting aspect of this famed bodybuilder’s story.
If you ever have the chance to hear him in an interview, your
biggest impression will be not the strength of his muscles but the
strength of his mind. La Lanne is mentally alert, almost beyond
reason. His sense of humor is both lightening fast and improvisatory.
“I tell people I can’t afford to die. It will wreck my image!” he once
exclaimed to Larry King. He regularly rails at the camera: “Why am I
so strong? Do you know how many calories are in butter and cheese
and ice cream? Would you get your dog up in the morning for a cup of
coffee and a doughnut?” He claims he hasn’t had dessert since 1929.
He is hyper-energized, opinionated, possessed with the intellectual
vigor of an athlete in his 20s.
So it’s hard not to ask: “Is there a relationship between exercise
and mental alertness?” The answer, it turns out, is yes.
survival of the fittest
Though a great deal of our evolutionary history remains shrouded
in controversy, the one fact that every paleoanthropologist on the
planet accepts can be summarized in two words:
We moved.
A lot. When our bountiful rainforests began to shrink, collapsing
the local food supply, we were forced to wander around an
increasingly dry landscape looking for more trees we could scamper
up to dine. As the climate got more arid, these wet botanical vending
machines disappeared altogether. Instead of moving up and down
complex arboreal environments in three dimensions, which required
a lot of dexterity, we began walking back and forth across arid
savannahs in two dimensions, which required a lot of stamina.
“About 10 to 20 kilometers a day with men,” says famed
anthropologist Richard Wrangham, “and about half that for women.”

19. 1. EXERCISE
11
That’s the amount of ground scientists estimate we covered on a daily
basis back then—up to 12 miles a day. That means our fancy brains
developed not while we were lounging around but while we were
working out.
The first real marathon runner of our species was a vicious
predator known as Homo erectus. As soon as the Homo erectus family
evolved, about 2 million years ago, he started moving out of town.
Our direct ancestors, Homo sapiens, rapidly did the same thing,
starting in Africa 100,000 years ago and reaching Argentina by
12,000 years ago. Some researchers suggest that we were extending
our ranges by an unheard-of 25 miles per year.
This is an impressive feat, considering the nature of the world our
ancestors inhabited. They were crossing rivers and deserts, jungles
and mountain ranges, all without the aid of maps and mostly without
tools. They eventually made ocean-going boats without the benefit
of wheels or metallurgy, and then traveling up and down the Pacific
with only the crudest navigational skills. Our ancestors constantly
were encountering new food sources, new predators, new physical
dangers. Along the road they routinely suffered injuries, experienced
strange illnesses, and delivered and nurtured children, all without
the benefit of textbooks or modern medicine.
Given our relative wimpiness in the animal kingdom (we don’t
even have enough body hair to survive a mildly chilly night), what
these data tell us is that we grew up in top physical shape, or we
didn’t grow up at all. And they also tell us the human brain became
the most powerful in the world under conditions where motion was
a constant presence.
If our unique cognitive skills were forged in the furnace of
physical activity, is it possible that physical activity still influences
our cognitive skills? Are the cognitive abilities of someone in good
physical condition different from those of someone in poor physical
condition? And what if someone in poor physical condition were
whipped into shape? Those are scientifically testable questions. The

20. BRAIN RULES
12
answers are directly related to why Jack La Lanne can still crack jokes
about eating dessert. In his nineties.
will you age like jim or like frank?
We discovered the beneficial effects of exercise on the brain
by looking at aging populations. This was brought home to me by
an anonymous man named Jim and a famous man named Frank. I
met them both while I was watching television. A documentary on
American nursing homes showed people in wheelchairs, many in
their mid- to late 80s, lining the halls of a dimly lit facility, just sitting
around, seemingly waiting to die. One was named Jim. His eyes
seemed vacant, lonely, friendless. He could cry at the drop of a hat
but otherwise spent the last years of his life mostly staring off into
space. I switched channels. I stumbled upon a very young-looking
Mike Wallace. The journalist was busy interviewing architect Frank
Lloyd Wright, at the time in his late 80s. I was about to hear a most
riveting interview.
“When I walk into St. Patrick’s Cathedral … here in New York
City, I am enveloped in a feeling of reverence,” said Wallace, tapping
his cigarette. The old man eyed Wallace. “Sure it isn’t an inferiority
complex?”
“Just because the building is big and I’m small, you mean?”
“Yes.”
“I think not.”
“I hope not.”
“You feel nothing when you go into St. Patrick’s?”
“Regret,” Wright said without a moment’s pause, “because it isn’t
the thing that really represents the spirit of independence and the
sovereignty of the individual which I feel should be represented in
our edifices devoted to culture.”
I was dumbfounded by the dexterity of Wright’s response. In four
sentences, one could detect the clarity of his mind, his unshakable
vision, his willingness to think out of the box. The rest of his

21. 1. EXERCISE
13
interview was just as compelling, as was the rest of Wright’s life. He
completed the designs for the Guggenheim Museum, his last work,
in 1957, when he was 90 years old.
But I also was dumbfounded by something else. As I
contemplated Wright’s answers, I remembered Jim from the nursing
home. He was the same age as Wright. In fact, most of the residents
were. I suddenly was beholding two types of aging. Jim and Frank
lived in roughly the same period of time. But one mind had almost
completely withered, while the other remained as incandescent as
a light bulb. What was the difference in the aging process between
men like Jim and the famous architect? This question has bugged the
research community for a long time. Investigators have known for
years that some people age with energy and pizazz, living productive
lives well into their 80s and 90s. Others appear to become battered
and broken by the process, and often they don’t survive their
70s. Attempts to explain these differences led to many important
discoveries, which I have grouped as answers to six questions.
1) Is there one factor that predicts how well you will age?
It was never an easy question for researchers to answer. They
found many variables, from nature to nurture, that contributed
to someone’s ability to age gracefully. That’s why the scientific
community met with both applause and suspicion a group of
researchers who uncovered a powerful environmental influence. In
a result that probably produced a smile on Jack La Lanne’s face, one
of the greatest predictors of successful aging was the presence or
absence of a sedentary lifestyle. Put simply, if you are a couch potato,
you are more likely to age like Jim, if you make it to your 80s at all.
If you have an active lifestyle, you are more likely to age like Frank
Lloyd Wright and much more likely to make it to your 90s.
The chief reason for the difference seemed to be that exercise
improved cardiovascular fitness, which in turn reduced the risk for
diseases such as heart attacks and stroke. But researchers wondered

22. BRAIN RULES
14
why the people who were aging “successfully” also seemed to be
more mentally alert. This led to the obvious second question:
2) Were they?
Just about every mental test possible was tried. No matter how
it was measured, the answer was consistently yes: A lifetime of
exercise can result in a sometimes astonishing elevation in cognitive
performance, compared with those who are sedentary. Exercisers
outperform couch potatoes in tests that measure long-term
memory, reasoning, attention, problem-solving, even so-called fluid-
intelligence tasks. These tasks test the ability to reason quickly and
think abstractly, improvising off previously learned material in order
to solve a new problem. Essentially, exercise improves a whole host
of abilities prized in the classroom and at work.
Not every weapon in the cognitive arsenal is improved by
exercise. Short-term memory skills, for example, and certain types of
reaction times appear to be unrelated to physical activity. And, while
nearly everybody shows some improvement, the degree of benefit
varies quite a bit among individuals. Most important, these data,
strong as they were, showed only an association, not a cause. To show
the direct link, a more intrusive set of experiments had to be done.
Researchers had to ask:
3) Can you turn Jim into Frank?
The experiments were reminiscent of a makeover show.
Researchers found a group of couch potatoes, measured their brain
power, exercised them for a period of time, and re-examined their
brain power. They consistently found that when couch potatoes are
enrolled in an aerobic exercise program, all kinds of mental abilities
begin to come back online. Positive results were observed after as
little as four months of activity. It was the same story with school-
age children. In one recent study, children jogged for 30 minutes two
or three times a week. After 12 weeks, their cognitive performance

23. 1. EXERCISE
15
had improved significantly compared with pre-jogging levels. When
the exercise program was withdrawn, the scores plummeted back to
their pre-experiment levels. Scientists had found a direct link. Within
limits, it does appear that exercise can turn Jim into Frank, or at least
turn Jim into a sharper version of himself.
As the effects of exercise on cognition became increasingly
obvious, scientists began fine-tuning their questions. One of the
biggest—certainly one dearest to the couch-potato cohort—was:
What type of exercise must you do, and how much of it must be done
to get the benefit? I have both good news and bad news.
4) What’s the bad news?
Astonishingly, after years of investigation in aging populations,
the answer to the question of how much is not much. If all you do
is walk several times a week, your brain will benefit. Even couch
potatoes who fidget show increased benefit over those who do not
fidget. The body seems to be clamoring to get back to its hyperactive
Serengeti roots. Any nod toward this history, be it ever so small, is
met with a cognitive war whoop. In the laboratory, the gold standard
appears to be aerobic exercise, 30 minutes at a clip, two or three
times a week. Add a strengthening regimen and you get even more
cognitive benefit.
Of course, individual results vary, and no one should embark on a
rigorous program without consulting a physician. Too much exercise
and exhaustion can hurt cognition. The data merely point to the
fact that one should embark. Exercise, as millions of years traipsing
around the backwoods tell us, is good for the brain. Just how good
took everyone by surprise, as they answered the next question.
5) Can exercise treat brain disorders?
Given the robust effect of exercise on typical cognitive
performance, researchers wanted to know if it could be used to
treat atypical performance. What about diseases such as age-related

24. BRAIN RULES
16
dementia and its more thoroughly investigated cousin, Alzheimer’s
disease? What about affective disorders such as depression?
Researchers looked at both prevention and intervention. With
experiments reproduced all over the world, enrolling thousands of
people, often studied for decades, the results are clear. Your lifetime
risk for general dementia is literally cut in half if you participate in
leisure-time physical activity. Aerobic exercise seems to be the key.
With Alzheimer’s, the effect is even greater: Such exercise lowers
your odds of getting the disease by more than 60 percent.
How much exercise? Once again, a little goes a long way. The
researchers showed you have to participate in some form of exercise
just twice a week to get the benefit. Bump it up to a 20-minute walk
each day, and you can cut your risk of having a stroke—one of the
leading causes of mental disability in the elderly—by 57 percent.
The man most responsible for stimulating this line of inquiry
did not start his career wanting to be a scientist. He wanted to be an
athletics coach. His name is Dr. Steven Blair, and he looks uncannily
like Jason Alexander, the actor who portrayed George Costanza on
the old TV sitcom Seinfeld. Blair’s coach in high school, Gene Bissell,
once forfeited a football game after discovering that an official had
missed a call. Even though the league office balked, Bissell insisted
that his team be declared the loser, and the young Steven never
forgot the incident. Blair writes that this devotion to truth inspired
his undying admiration for rigorous, no-nonsense, statistical analysis
of the epidemiological work in which he eventually embarked. His
seminal paper on fitness and mortality stands as a landmark example
of how to do work with integrity in this field. The rigor of his findings
inspired other investigators. What about using exercise not only as
prevention, they asked, but as intervention, to treat mental disorders
such as depression and anxiety?
That turned out to be a good line of questioning. A growing body
of work now suggests that physical activity can powerfully affect the
course of both diseases. We think it’s because exercise regulates the

25. 1. EXERCISE
17
release of the three neurotransmitters most commonly associated
with the maintenance of mental health: serotonin, dopamine, and
norepinephrine. Although exercise cannot substitute for psychiatric
treatment, the role of exercise on mood is so pronounced that
many psychiatrists have begun adding a regimen of physical activity
to the normal course of therapy. But in one experiment with
depressed individuals, rigorous exercise was actually substituted for
antidepressant medication. Even when compared against medicated
controls, the treatment outcomes were astonishingly successful. For
both depression and anxiety, exercise is beneficial immediately and
over the long term. It is equally effective for men and women, and
the longer the program is deployed, the greater the effect becomes. It
is especially helpful for severe cases and for older people.
Most of the data we have been discussing concern elderly
populations. Which leads to the question:
6) Are the cognitive blessings of exercise only for the elderly?
As you ratchet down the age chart, the effects of exercise on
cognition become less clear. The biggest reason for this is that so
few studies have been done. Only recently has the grumpy scientific
eye begun to cast its gaze on younger populations. One of the best
efforts enrolled more than 10,000 British civil servants between the
ages of 35 and 55, examining exercise habits and grading them as
low, medium, or high. Those with low levels of physical activity were
more likely to have poor cognitive performance. Fluid intelligence,
the type that requires improvisatory problem-solving skills, was
particularly hurt by a sedentary lifestyle. Studies done in other
countries have confirmed the finding.
If only a small number of studies have been done in middle-age
populations, the number of studies saying anything about exercise
and children is downright microscopic. Though much more work
needs to be done, the data point in a familiar direction, though
perhaps for different reasons.

26. BRAIN RULES
18
To talk about some of these differences, I would like to introduce
you to Dr. Antronette Yancey. At 6 foot 2, Yancey is a towering,
beautiful presence, a former professional model, now a physician-
scientist with a deep love for children and a broad smile to buttress
the attitude. She is a killer basketball player, a published poet, and
one of the few professional scientists who also makes performance
art. With this constellation of talents, she is a natural to study the
effects of physical activity on developing minds. And she has
found what everybody else has found: Exercise improves children.
Physically fit children identify visual stimuli much faster than
sedentary ones. They appear to concentrate better. Brain-activation
studies show that children and adolescents who are fit allocate more
cognitive resources to a task and do so for longer periods of time.
“Kids pay better attention to their subjects when they’ve been
active,” Yancey says. “Kids are less likely to be disruptive in terms of
their classroom behavior when they’re active. Kids feel better about
themselves, have higher self-esteem, less depression, less anxiety. All
of those things can impair academic performance and attentiveness.”
Of course, there are many ingredients to the recipe of academic
performance. Finding out which components are the most
important—especially if you want improvement—is difficult enough.
Finding out whether exercise is one of those choice ingredients is
even tougher. But these preliminary findings show that we have
every reason to be optimistic about the long-term outcomes.
an exercise in road-building
Why exercise works so well in the brain, at a molecular level,
can be explained by competitive food eaters—or, less charitably,
professional pigs. There is an international association representing
people who time themselves on how much they can eat at a given
event. The association is called the International Federation of
Competitive Eating, and its crest proudly displays the slogan (I am
not making this up) In Voro Veritas—literally, “In Gorging, Truth.”

27. 1. EXERCISE
19
Like any sporting organization, competitive food eaters have their
heroes. The reigning gluttony god is Takeru “Tsunami” Kobayashi.
He is the recipient of many eating awards, including the vegetarian
dumpling competition (83 dumplings downed in 8 minutes),
the roasted pork bun competition (100 in 12 minutes), and the
hamburger competition (97 in 8 minutes). Kobayashi also is a world
champion hot-dog eater. One of his few losses was to a 1,089-pound
Kodiak bear. In a 2003 Fox televised special called Man vs. Beast, the
mighty Kobayashi consumed only 31 bunless dogs compared with
the ursine’s 50, all in about 2½ minutes. Kobayashi lost his hot-dog
crown in 2007 to Joey Chestnut, who ate 66 hot dogs in 12 minutes
(the Tsunami could manage only 63).
But my point isn’t about speed. It’s about what happens to all of
those hot dogs after they slide down the Tsunami’s throat. As with
any of us, his body uses its teeth and acid and wormy intestines to
tear the food apart and, if need be, reconfigure it.
This is done for more or less a single reason: to turn foodstuffs
into glucose, a type of sugar that is one of the body’s favorite energy
resources. Glucose and other metabolic products are absorbed into
the bloodstream via the small intestines. The nutrients travel to all
parts of the body, where they are deposited into cells, which make
up the body’s various tissues. The cells seize the sweet stuff like
sharks in a feeding frenzy. Cellular chemicals greedily tear apart
the molecular structure of glucose to extract its sugary energy. This
energy extraction is so violent that atoms are literally ripped asunder
in the process.
As in any manufacturing process, such fierce activity generates
a fair amount of toxic waste. In the case of food, this waste consists
of a nasty pile of excess electrons shredded from the atoms in the
glucose molecules. Left alone, these electrons slam into other
molecules within the cell, transforming them into some of the most
toxic substances known to humankind. They are called free radicals.
If not quickly corralled, they will wreck havoc on the innards of a cell

28. BRAIN RULES
20
and, cumulatively, on the rest of the body. These electrons are fully
capable, for example, of causing mutations in your very DNA.
The reason you don’t die of electron overdose is that the
atmosphere is full of breathable oxygen. The main function of oxygen
is to act like an efficient electron-absorbing sponge. At the same time
the blood is delivering foodstuffs to your tissues, it is also carrying
these oxygen sponges. Any excess electrons are absorbed by the
oxygen and, after a bit of molecular alchemy, are transformed into
equally hazardous—but now fully transportable—carbon dioxide. The
blood is carried back to your lungs, where the carbon dioxide leaves
the blood and you breathe it out. So, whether you are a competitive
eater or a typical one, the oxygen-rich air you inhale keeps the food
you eat from killing you.
Getting food into tissues and getting toxic electrons out obviously
are matters of access. That’s why blood has to be everywhere inside
you. Serving as both wait staff and haz-mat team, any tissue without
enough blood supply is going to starve to death—your brain included.
That’s important because the brain’s appetite for energy is enormous.
The brain represents only about 2 percent of most people’s body
weight, yet it accounts for about 20 percent of the body’s total energy
usage—about 10 times more than would be expected. When the
brain is fully working, it uses more energy per unit of tissue weight
than a fully exercising quadricep. In fact, the human brain cannot
simultaneously activate more than 2 percent of its neurons at any
one time. More than this, and the glucose supply becomes so quickly
exhausted that you will faint.
If it sounds to you like the brain needs a lot of glucose—and
generates a lot of toxic waste—you are right on the money. This
means the brain also needs lots of oxygen-soaked blood. How
much food and waste can the brain generate in just a few minutes?
Consider the following statistics. The three requirements for human
life are food, drink, and fresh air. But their effects on survival have
very different timelines. You can live for 30 days or so without food,

29. 1. EXERCISE
21
and you can go for a week or so without drinking water. Your brain,
however, is so active that it cannot go without oxygen for more than
5 minutes without risking serious and permanent damage. Toxic
electrons over-accumulate because the blood can’t deliver enough
oxygen sponges. Even in a healthy brain, the blood’s delivery system
can be improved. That’s where exercise comes in. It reminds me of a
seemingly mundane little insight that literally changed the history of
the world.
The man with the insight was named John Loudon McAdam.
McAdam, a Scottish engineer living in England in the early 1800s,
noticed the difficulty people had trying to move goods and supplies
over hole-filled, often muddy, frequently impassable dirt roads. He
got the splendid idea of raising the level of the road using layers of
rock and gravel. This immediately made the roads more stable, less
muddy, and less flood-prone. As county after county adopted his
process, now called macadamization, an astonishing after-effect
occurred. People instantly got more dependable access to one
another’s goods and services. Offshoots from the main roads sprang
up, and pretty soon entire countrysides had access to far-flung points
using stable arteries of transportation. Trade grew. People got richer.
By changing the way things moved, McAdam changed the way we
lived. What does this have to do with exercise? McAdam’s central
notion wasn’t to improve goods and services, but to improve access to
goods and services. You can do the same for your brain by increasing
the roads in your body, namely your blood vessels, through exercise.
Exercise does not provide the oxygen and the food. It provides your
body greater access to the oxygen and the food. How this works is
easy to understand.
When you exercise, you increase blood flow across the tissues
of your body. This is because exercise stimulates the blood vessels
to create a powerful, flow-regulating molecule called nitric oxide.
As the flow improves, the body makes new blood vessels, which
penetrate deeper and deeper into the tissues of the body. This allows

30. BRAIN RULES
22
more access to the bloodstream’s goods and services, which include
food distribution and waste disposal. The more you exercise, the
more tissues you can feed and the more toxic waste you can remove.
This happens all over the body. That’s why exercise improves the
performance of most human functions. You stabilize existing
transportation structures and add new ones, just like McAdam’s
roads. All of a sudden, you are becoming healthier.
The same happens in the human brain. Imaging studies have
shown that exercise literally increases blood volume in a region of the
brain called the dentate gyrus. That’s a big deal. The dentate gyrus
is a vital constituent of the hippocampus, a region deeply involved
in memory formation. This blood-flow increase, which may be the
result of new capillaries, allows more brain cells greater access to the
blood’s food and haz-mat teams.
Another brain-specific effect of exercise recently has become
clear, one that isn’t reminiscent of roads so much as of fertilizer.
At the molecular level, early studies indicate that exercise also
stimulates one of the brain’s most powerful growth factors, BDNF.
That stands for Brain Derived Neurotrophic Factor, and it aids in the
development of healthy tissue. BDNF exerts a fertilizer-like growth
effect on certain neurons in the brain. The protein keeps existing
neurons young and healthy, rendering them much more willing
to connect with one another. It also encourages neurogenesis, the
formation of new cells in the brain. The cells most sensitive to this
are in the hippocampus, inside the very regions deeply involved in
human cognition. Exercise increases the level of usable BDNF inside
those cells. The more you exercise, the more fertilizer you create—at
least, if you are a laboratory animal. There are now suggestions that
the same mechanism also occurs in humans.
we can make a comeback
All of the evidence points in one direction: Physical activity
is cognitive candy. We can make a species-wide athletic comeback.

31. 1. EXERCISE
23
All we have to do is move. When people think of great comebacks,
athletes such as Lance Armstrong or Paul Hamm usually come to
mind. One of the greatest comebacks of all time, however, occurred
before both of these athletes were born. It happened in 1949 to the
legendary golfer Ben Hogan.
Prickly to the point of being obnoxious (he once quipped of
a competitor, “If we could have just screwed another head on his
shoulders, he would have been the greatest golfer who ever lived”),
Hogan’s gruff demeanor underscored a fierce determination. He
won the PGA championship in 1946 and in 1948, the year in which
he was also named PGA Player of the Year. That all ended abruptly.
On a foggy night in the Texas winter of 1949, Hogan and his wife
were hit head-on by a bus. Hogan fractured every bone that could
matter to a golfer: collar bone, pelvis, ankle, rib. He was left with life-
threatening blood clots. The doctors said he might never walk again,
let alone play golf. Hogan ignored their prognostications. A year after
the accident, he climbed back onto the green and won the U.S. Open.
Three years later, he played one of the most successful single seasons
in professional golf. He won five of the six tournaments he entered,
including the first three major championships of the year (a feat
now known as the Hogan Slam). Reflecting on one of the greatest
comebacks in sports history, he said in his typically spicy manner,
“People have always been telling me what I can’t do.” He retired in
1971.
When I reflect on the effects of exercise on cognition and the
things we might try to recapture its benefits, I am reminded of such
comebacks. Civilization, while giving us such seemingly forward
advances as modern medicine and spatulas, also has had a nasty side
effect. It gave us more opportunities to sit on our butts. Whether
learning or working, we gradually quit exercising the way our
ancestors did. The result is like a traffic wreck.
Recall that our evolutionary ancestors were used to walking up to
12 miles per day. This means that our brains were supported for most

32. BRAIN RULES
24
of our evolutionary history by Olympic-caliber bodies. We were not
used to sitting in a classroom for 8 hours at a stretch. We were not
used to sitting in a cubicle for 8 hours at a stretch. If we sat around
the Serengeti for 8 hours—heck, for 8 minutes—we were usually
somebody’s lunch. We haven’t had millions of years to adapt to our
sedentary lifestyle. That means we need a comeback. Removing
ourselves from such inactivity is the first step. I am convinced that
integrating exercise into those 8 hours at work or school will not
make us smarter. It will only make us normal.
ideas
There is no question we are in an epidemic of fatness, a point I
will not belabor here. The benefits of exercise seem nearly endless
because its impact is systemwide, affecting most physiological
systems. Exercise makes your muscles and bones stronger, for
example, and improves your strength and balance. It helps regulate
your appetite, changes your blood lipid profile, reduces your risk for
more than a dozen types of cancer, improves the immune system,
and buffers against the toxic effects of stress (see Chapter 8). By
enriching your cardiovascular system, exercise decreases your risk
for heart disease, stroke, and diabetes. When combined with the
intellectual benefits exercise appears to offer, we have in our hands
as close to a magic bullet for improving human health as exists
in modern medicine. There must be ways to harness the effects of
exercise in the practical worlds of education and business.
Recess twice a day
Because of the increased reliance on test scores for school
survival, many districts across the nation are getting rid of physical
education and recess. Given the powerful cognitive effects of physical
activity, this makes no sense. Yancey, the model-turned-physican/
scientist/basketball player, describes a real-world test:
“They took time away from academic subjects for physical

33. 1. EXERCISE
25
education … and found that, across the board, [physical education]
did not hurt the kids’ performance on the academic tests. … [When]
trained teachers provided the physical education, the children
actually did better on language, reading and the basic battery of
tests.”
Cutting off physical exercise—the very activity most likely to
promote cognitive performance—to do better on a test score is like
trying to gain weight by starving yourself. What if a school district
inserted exercise into the normal curriculum on a regular basis, even
twice a day? After all of the children had been medically evaluated,
they’d spend 20 to 30 minutes each morning on formal aerobic
exercise; in the afternoon, 20 to 30 minutes on strengthening
exercises. Most populations studied see a benefit if this is done
only two or three times a week. If it worked, there would be many
ramifications. It might even reintroduce the notion of school
uniforms. Of what would the new apparel consist? Simply gym
clothes, worn all day long.
Treadmills in classrooms and cubicles
Remember the experiment showing that when children
aerobically exercised, their brains worked better, and when the
exercise was withdrawn, the cognitive gain soon plummeted? These
results suggested to the researchers that the level of fitness was not
as important as a steady increase in the oxygen supply to the brain
(otherwise the improved mental sharpness would not have fallen
off so rapidly). So they did another experiment. They found that
supplemental oxygen administered to young healthy adults without
exercise gave a similar cognitive improvement.
This suggests an interesting idea to try in a classroom (don’t
worry, it doesn’t involve oxygen doping to get a grade boost). What
if, during a lesson, the children were not sitting at desks but walking
on treadmills? Students might listen to a math lecture while walking
1 to 2 miles per hour, or study English on treadmills fashioned to

34. BRAIN RULES
26
accommodate a desktop. Treadmills in the classroom might harness
the valuable advantages of increasing the oxygen supply naturally and
at the same time harvest all the other advantages of regular exercise.
Would such a thing, deployed over a school year, change academic
performance? Until brain scientists and education scientists get
together to show real-world benefit, the answer is: Nobody knows.
The same idea could apply at work, with companies installing
treadmills and encouraging morning and afternoon breaks for
exercise. Board meetings might be conducted while people walked
2 miles per hour. Would that improve problem-solving? Would it
alter retention rates or change creativity the same way it does in the
laboratory?
The idea of integrating exercise into the workday may sound
foreign, but it’s not difficult. I put a treadmill in my own office, and
I now take regular breaks filled not with coffee but with exercise. I
even constructed a small structure upon which my laptop fits so I can
write email while I exercise. At first, it was difficult to adapt to such a
strange hybrid activity. It took a whopping 15 minutes to become fully
functional typing on my laptop while walking 1.8 miles per hour.
I’m not the only one thinking along these lines. Boeing, for
example, is starting to take exercise seriously in its leadership-
training programs. Problem-solving teams used to work late into the
night; now, all work has to be completed during the day so there’s
time for exercise and sleep. More teams are hitting all of their
performance targets. Boeing’s vice president of leadership has put a
treadmill in her office as well, and she reports that the exercise clears
her mind and helps her focus. Company leaders are now thinking
about how to integrate exercise into working hours.
There are two compelling business reasons for such radical ideas.
Business leaders already know that if employees exercised regularly, it
would reduce health-care costs. And there’s no question that cutting
in half someone’s lifetime risk of a debilitating stroke or Alzheimer’s
disease is a wonderfully humanitarian thing to do. But exercise

35. 1. EXERCISE
27
also could boost the collective brain power of an organization. Fit
employees are capable of mobilizing their God-given IQs better
than sedentary employees. For companies whose competitiveness
rests on creative intellectual horsepower, such mobilization could
mean a strategic advantage. In the laboratory, regular exercise
improves—sometimes dramatically so—problem-solving abilities,
fluid intelligence, even memory. Would it do so in business settings?
What types of exercise need to be done, and how often? That’s worth
investigating.

36. Summary
Rule #1
Exercise boosts brain power.
Our brains were built for walking—12 miles a day!
•
•
To improve your thinking skills,
•
• move.
Exercise gets blood to your brain, bringing it glucose
•
•
for energy and oxygen to soak up the toxic electrons
that are left over. It also stimulates the protein that keeps
neurons connecting.
Aerobic exercise just twice a week halves your risk of
•
•
general dementia. It cuts your risk of Alzheimer’s by 60
percent.
Get illustrations, audio, video, and more
at www.brainrules.net

37. survival
Rule #2
The human brain evolved, too.

39. When he was 4, my son Noah picked up a stick in our
backyard and showed it to me. “Nice stick you have
there, young fellow,” I said. He replied earnestly, “That’s
not a stick. That’s a sword! Stick ’em up!” And I raised my
hands to the air. We both laughed. The reason I remember this short
exchange is that as I went back into the house, I realized my son
had just displayed virtually every unique thinking ability a human
possesses—one that took several million years to manufacture. And
he did so in less than two seconds.
Heavy stuff for a 4-year-old. Other animals have powerful
cognitive abilities, too, and yet there is something qualitatively
different about the way humans think about things. The journey that
brought us from the trees to the savannah gave us some structural
elements shared by no other creature—and unique ways of using the
elements we do have in common. How and why did our brains evolve
this way?
Recall the performance envelope: The brain appears to be

40. BRAIN RULES
32
designed to (1) solve problems (2) related to surviving (3) in an
unstable outdoor environment, and (4) to do so in nearly constant
motion. The brain adapted this way simply as a survival strategy, to
help us live long enough to pass our genes on to the next generation.
That’s right: It all comes down to sex. Ecosystems are harsh, crushing
life as easily as supporting it. Scientists estimate 99.99% of all species
that have ever lived are extinct today. Our bodies, brains included,
latched on to any genetic adaptation that helped us survive. This not
only sets the stage for all of the Brain Rules, it explains how we came
to conquer the world.
There are two ways to beat the cruelty of the environment: You
can become stronger or you can become smarter. We chose the
latter. It seems most improbable that such a physically weak species
could take over the planet not by adding muscles to our skeletons
but by adding neurons to our brains. But we did, and scientists have
spent a great deal of effort trying to figure out how. Judy DeLoache
has studied this question extensively. She became a well-respected
researcher in an era when women were actively discouraged from
studying investigative science, and she is still going strong at the
University of Virginia. Her research focus, given her braininess?
Appropriately, it is human braininess itself. She is especially
interested in how human cognition can be distinguished from the
way other animals think about their respective worlds.
One of her major contributions was to identify the human
trait that really does separate us from the gorillas: the ability to
use symbolic reasoning. That’s what my son was doing when he
brandished his stick sword. When we see a five-sided geometric
shape, we’re not stuck perceiving it as a pentagon. We can just as
easily perceive the U.S. military headquarters. Or a Chrysler minivan.
Our brain can behold a symbolic object as real all by itself and yet,
simultaneously, also representing something else. Maybe somethings
else. DeLoache calls it Dual Representational Theory. Stated formally,
it describes our ability to attribute characteristics and meanings to

41. 2. SURVIVAL
33
things that don’t actually possess them. Stated informally, we can
make things up that aren’t there. We are human because we can
fantasize.
Draw a vertical line in your hand. Does it have to stay a vertical
line? Not if you know how to impute a characteristic onto something
it does not intrinsically possess. Go ahead and put a horizontal line
under it. Now you have the number 1. Put a dot on the top of it. Now
you have the letter “i.” The line doesn’t have to mean a line. The line
can mean anything you darn well think it should mean. The meaning
can become anchored to a symbol simply because it is not forced to
become anchored to anything else. The only thing you have to do is
get everybody to agree on what a symbol should mean.
We are so good at dual representation, we combine symbols to
derive layers of meaning. It gives us the capacity for language, and
for writing down that language. It gives us the capacity to reason
mathematically. It gives us the capacity for art. Combinations
of circles and squares become geometry and Cubist paintings.
Combinations of dots and squiggles become music and poetry. There
is an unbroken intellectual line between symbolic reasoning and
the ability to create culture. And no other creature is capable of doing
it.
This ability isn’t fully formed at birth. DeLoache was able to
show this in a powerful way. In DeLoache’s laboratory, a little girl
plays with a dollhouse. Next door is an identical room, but life-size.
DeLoache takes a small plastic dog and puts it under the dollhouse
couch, then encourages the child to go into the “big” living room
next door and find a “big” version of the dog. What does the little
girl do? If she is 36 months of age, DeLoache found, she immediately
goes to the big room, looks under the couch, and finds the big dog.
But if the child is 30 months old, she has no idea where to look. She
cannot reason symbolically and cannot connect the little room with
the big room. Exhaustive studies show that symbolic reasoning, this
all-important human trait, takes almost three years of experience to

42. BRAIN RULES
34
become fully operational. We don’t appear to do much to distinguish
ourselves from apes before we are out of the terrible twos.
a handy trait
Symbolic reasoning turned out to be a versatile gadget. Our
evolutionary ancestors didn’t have to keep falling into the same
quicksand pit if they could tell others about it; even better if they
learned to put up warning signs. With words and language, we could
extract a great deal of knowledge about our living situation without
always having to experience its harsh lessons directly. So it makes
sense that once our brains developed symbolic reasoning, we kept it.
The brain is a biological tissue; it follows the rules of biology. And
there’s no bigger rule in biology than evolution through natural
selection: Whoever gets the food survives; whoever survives gets to
have sex; and whoever has sex gets to pass his traits on to the next
generation. But what stages did we go through to reach that point?
How can we trace the rise of our plump, 3-pound intellects?
You might remember those old posters showing the development
of humankind as a series of linear and increasingly sophisticated
creatures. I have an old one in my office. The first drawing shows
a chimpanzee; the final drawing shows a 1970s businessman. In
between are strangely blended variations of creatures with names
like Peking man and Australopithecus. There are two problems with
this drawing. First, almost everything about it is wrong. Second,
nobody really knows how to fix the errors. One of the biggest reasons
for our lack of knowledge is that so little hard evidence exists. Most
of the fossilized bones that have been collected from our ancestors
could fit into your garage, with enough room left over for your
bicycle and lawn mower. DNA evidence has been helpful, and there
is strong evidence that we came from Africa somewhere between 7
million and 10 million years ago. Virtually everything else is disputed
by some cranky professional somewhere.
Understanding our intellectual progress has been just as difficult.

43. 2. SURVIVAL
35
Most of it has been charted by using the best available evidence:
tool-making. That’s not necessarily the most accurate way; even if
it were, the record is not very impressive. For the first few million
years, we mostly just grabbed rocks and smashed them into things.
Scientists, perhaps trying to salvage some of our dignity, called
these stones hand axes. A million years later, our progress still was
not very impressive. We still grabbed “hand axes,” but we began to
smash them into other rocks, making them more pointed. Now we
had sharper rocks.
It wasn’t much, but it was enough to begin untethering ourselves
from our East African womb, and indeed any other ecological niche.
Things got more impressive, from creating fire to cooking food.
Eventually, we migrated out of Africa in successive waves, our first
direct Homo sapien ancestors making the journey as little as 100,000
years ago. Then, 40,000 years ago, something almost unbelievable
happened. They appeared suddenly to have taken up painting and
sculpture, creating fine art and jewelry. No one knows why the
changes were so abrupt, but they were profound. Thirty-seven
thousand years later, we were making pyramids. Five thousand years
after that, rocket fuel.
What happened to start us on our journey? Could the growth
spurt be explained by the onset of dual-representation ability? The
answer is fraught with controversy, but the simplest explanation is
by far the clearest. It seems our great achievements mostly had to do
with a nasty change in the weather.
new rules for survival
Most of human prehistory occurred in climates like the jungles of
South America: steamy, humid, and in dire need of air conditioning.
It was comfortably predictable. Then the climate changed. Scientists
estimate that there have been no fewer than 17 Ice Ages in the past
40 million years. Only in a few places, such as the Amazonian and
African rainforests, does anything like our original, sultry, millions-

44. BRAIN RULES
36
of-years-old climate survive. Ice cores taken from Greenland
show that the climate staggers from being unbearably hot to being
sadistically cold. As little as 100,000 years ago, you could be born in
a nearly arctic environment but then, mere decades later, be taking
off your loincloth to catch the golden rays of the grassland sun.
Such instability was bound to have a powerful effect on any
creature forced to endure it. Most could not. The rules for survival
were changing, and a new class of creatures would start to fill the
vacuum created as more and more of their roommates died out.
That was the crisis our ancestors faced as the tropics of Northern
and Eastern Africa turned to dry, dusty plains—not immediately, but
inexorably—beginning maybe 10 million years ago. Some researchers
blame it on the Himalayas, which had reached such heights as to
disturb global atmospheric currents. Others blame the sudden
appearance of the Isthmus of Panama, which changed the mixing of
the Pacific and Atlantic ocean currents and disturbed global weather
patterns, as El Niños do today.
Whatever the reason, the changes were powerful enough to
disrupt the weather all over the world, including in our African
birthplace. But not too powerful, or too subtle—a phenomenon
called the Goldilocks Effect. If the changes had been too sudden,
the climatic violence would have killed our ancestors outright, and I
wouldn’t be writing this book for you today. If the changes had been
too slow, there may have been no reason to develop our talent for
symbolism and, once again, no book. Instead, like Goldilocks and the
third bowl of porridge, the conditions were just right. The change
was enough to shake us out of our comfortable trees, but it wasn’t
enough to kill us when we landed.
Landing was only the beginning of the hard work, however. We
quickly discovered that our new digs were already occupied. The
locals had co-opted the food sources, and most of them were stronger
and faster than we were. Faced with grasslands rather than trees,
we rudely were introduced to the idea of “flat.” It is disconcerting

45. 2. SURVIVAL
37
to think that we started our evolutionary journey on an unfamiliar
horizontal plane with the words “Eat me, I’m prey” taped to the back
of our evolutionary butts.
jazzin’ on a riff
You might suspect that the odds against our survival were great.
You would be right. The founding population of our direct ancestors
is not thought to have been much larger than 2,000 individuals;
some think the group was as small as a few hundred. How, then, did
we go from such a wobbly, fragile minority population to a staggering
tide of humanity 7 billion strong and growing? There is only one way,
according to Richard Potts, director of the Human Origins Program
at the Smithsonian’s National Museum of Natural History. You give
up on stability. You don’t try to beat back the changes. You begin
not to care about consistency within a given habitat, because such
consistency isn’t an option. You adapt to variation itself.
It was a brilliant strategy. Instead of learning how to survive in
just one or two ecological niches, we took on the entire globe. Those
unable to rapidly solve new problems or learn from mistakes didn’t
survive long enough to pass on their genes. The net effect of this
evolution was that we didn’t become stronger; we became smarter.
We learned to grow our fangs not in the mouth but in the head. This
turned out to be a pretty savvy strategy. We went on to conquer the
small rift valleys in Eastern Africa. Then we took over the world.
Potts calls his notion Variability Selection Theory, and it
attempts to explain why our ancestors became increasingly allergic
to inflexibility and stupidity. Little in the fossil record is clear about
the exact progression—another reason for bitter controversy—but
all researchers must contend with two issues. One is bipedalism; the
other has to do with our increasingly big heads.
Variability Selection Theory predicts some fairly simple things
about human learning. It predicts there will be interactions between
two powerful features of the brain: a database in which to store a

46. BRAIN RULES
38
fund of knowledge, and the ability to improvise off of that database.
One allows us to know when we’ve made mistakes. The other
allows us to learn from them. Both give us the ability to add new
information under rapidly changing conditions. Both may be relevant
to the way we design classrooms and cubicles.
Any learning environment that deals with only the database
instincts or only the improvisatory instincts ignores one half of our
ability. It is doomed to fail. It makes me think of jazz guitarists:
They’re not going to make it if they know a lot about music theory
but don’t know how to jam in a live concert. Some schools and
workplaces emphasize a stable, rote-learned database. They ignore
the improvisatory instincts drilled into us for millions of years.
Creativity suffers. Others emphasize creative usage of a database,
without installing a fund of knowledge in the first place. They ignore
our need to obtain a deep understanding of a subject, which includes
memorizing and storing a richly structured database. You get people
who are great improvisers but don’t have depth of knowledge. You
may know someone like this where you work. They may look like jazz
musicians and have the appearance of jamming, but in the end they
know nothing. They’re playing intellectual air guitar.
standing tall
Variability Selection Theory allows a context for dual
representation, but it hardly gets us to the ideas of Judy DeLoache
and our unique ability to invent calculus and write romance novels.
After all, many animals create a database of knowledge, and many
of them make tools, which they even use creatively. Still, it is not
as if chimpanzees write symphonies badly and we write them
well. Chimps can’t write them at all, and we can write ones that
make people spend their life savings on subscriptions to the New
York Philharmonic. There must have been something else in our
evolutionary history that made human thinking unique.
One of the random genetic mutations that gave us an adaptive

47. 2. SURVIVAL
39
advantage involved learning to walk upright. The trees were gone
or going, so we had to deal with something new in our experience:
walking increasingly long distances between food sources. That
eventually involved the specialized use of our two legs. Bipedalism
was an excellent solution to a vanishing rainforest. But it was also a
major change. At the very least, it meant refashioning the pelvis so
that it no longer propelled the back legs forward (which is what it
does for great apes). Instead, the pelvis had to be re-imagined as a
load-bearing device capable of keeping the head above the grass
(which is what it does for you). Walking on two legs had several
consequences. For one thing, it freed up our hands. For another, it
was energy-efficient. It used fewer calories than walking on four legs.
Our ancestral bodies used the energy surplus not to pump up our
muscles but to pump up our minds—to the point that our modern-
day brain, 2 percent of our body weight, sucks up 20 percent of the
energy we consume.
These changes in the structure of the brain led to the masterpiece
of evolution, the region that distinguishes humans from all other
creatures. It is a specialized area of the frontal lobe, just behind the
forehead, called the prefrontal cortex.
We got our first hints about its function from a man named
Phineas Gage, who suffered the most famous occupational injury
in the history of brain science. The injury didn’t kill him, but his
family probably wished it had. Gage was a popular foreman of a
railroad construction crew. He was funny, clever, hardworking, and
responsible, the kind of man any dad would be proud to call “son-in-
law.” On September 13, 1848, he set an explosives charge in the hole
of a rock using a tamping iron, a 3-foot rod about an inch in diameter.
The charge blew the rod into Gage’s head, entering just under the
eye and destroying most of his prefrontal cortex. Miraculously, Gage
survived, but he became tactless, impulsive, and profane. He left his
family and wandered aimlessly from job to job. His friends said he
was no longer Gage.

48. BRAIN RULES
40
This was the first real evidence that the prefrontal cortex
governs several uniquely human cognitive talents, called “executive
functions”: solving problems, maintaining attention, and inhibiting
emotional impulses. In short, this region controls many of the
behaviors that separate us from other animals. And from teenagers.
meet your brain
The prefrontal cortex is only the newest addition to the
brain. Three brains are tucked inside your head, and parts of their
structure took millions of years to design. (This “triune theory of the
brain” is one of several models scientists use to describe the brain’s
overarching structural organization.) Your most ancient neural
structure is the brain stem, or “lizard brain.” This rather insulting
label reflects the fact that the brain stem functions the same in you
as in a gila monster. The brain stem controls most of your body’s
housekeeping chores. Its neurons regulate breathing, heart rate,
sleeping, and waking. Lively as Las Vegas, they are always active,
keeping your brain purring along whether you’re napping or wide
awake.
Sitting atop your brain stem is what looks like a sculpture
of a scorpion carrying a slightly puckered egg on its back. The
Paleomammalian brain appears in you the same way it does in
many mammals, such as house cats, which is how it got its name.
It has more to do with your animal survival than with your human
potential. Most of its functions involve what some researchers
call the “four F’s”: fighting, feeding, fleeing, and … reproductive
behavior.
Several parts of this “second brain” play a large role in the Brain
Rules. The claw of the scorpion, called the amygdale, allows you
to feel rage. Or fear. Or pleasure. Or memories of past experiences
of rage, fear, or pleasure. The amygdala is responsible for both
the creation of emotions and the memories they generate. The
leg attaching the claw to the body of the scorpion is called the

49. 2. SURVIVAL
41
hippocampus. The hippocampus converts your short-term memories
into longer-term forms. The scorpion’s tail curls over the egg-
shaped structure like the letter “C,” as if protecting it. This egg is
the thalamus, one of the most active, well-connected parts of the
brain—a control tower for the senses. Sitting squarely in the center
of your brain, it processes signals sent from nearly every corner of
More illustrations at www.brainrules.net
you have three brains

50. BRAIN RULES
42
your sensory universe, then routes them to specific areas throughout
your brain.
How this happens is mysterious. Large neural highways run
overhead these two brains, combining with other roads, branching
suddenly into thousands of exits, bounding off into the darkness.
Neurons spark to life, then suddenly blink off, then fire again.
Complex circuits of electrical information crackle in coordinated,
repeated patterns, then run off into the darkness, communicating
their information to unknown destinations.
Arching above like a cathedral is your “human brain,” the cortex.
Latin for “bark,” the cortex is the surface of your brain. It is in deep
electrical communication with the interior. This “skin” ranges in
thickness from that of blotting paper to that of heavy-duty cardboard.
It appears to have been crammed into a space too small for its surface
area. Indeed, if your cortex were unfolded, it would be about the size
of a baby blanket.
It looks monotonous, slightly like the shell of a walnut, which
fooled anatomists for hundreds of years. Until World War I came
along, they had no idea each region of the cortex was highly
specialized, with sections for speech, for vision, for memory. World
War I was the first major conflict where large numbers of combatants
encountered shrapnel, and where medical know-how allowed them
to survive their injuries. Some of these injuries penetrated only to the
periphery of the brain, destroying tiny regions of cortex while leaving
everything else intact. Enough soldiers were hurt that scientists
could study in detail the injuries and the truly strange behaviors that
resulted. Horribly confirming their findings during World War II,
scientists eventually were able to make a complete structure-function
map of the brain—and see how it had changed over the eons.
They found that as our brains evolved, our heads did, too: They
were getting bigger all the time. Tilted hips and big heads are not
easy anatomical neighbors. The pelvis—and birth canal—can be
only so wide, which is bonkers if you are giving birth to children

51. 2. SURVIVAL
43
with larger and larger heads. A lot of mothers and babies died on the
way to reaching an anatomical compromise. Human pregnancies
are still remarkably risky without modern medical intervention.
The solution? Give birth while the baby’s head is small enough to
fit through the birth canal. The problem? You create childhood. The
brain could conveniently finish its developmental programs outside
the womb, but the trade-off was a creature vulnerable to predation
for years and not reproductively fit for more than a decade. That’s an
eternity if you make your living in the great outdoors, and outdoors
was our home address for eons. But it was worth it. During this time
of extreme vulnerability, you had a creature fully capable of learning
just about anything and, at least for the first few years, not good for
doing much else. This created the concept not only of learner but,
for adults, of teacher. It was in our best interests to teach well: Our
genetic survival depended upon our ability to protect the little ones.
Of course, it was no use having babies who took years to grow
if the adults were eaten before they could finish their thoughtful
parenting. Weaklings like us needed a tactic that could allow us to
outcompete the big boys in their home turf, leaving our new home
safer for sex and babies. We decided on a strange one. We decided to
try to get along with each other.
you scratch my back...
Suppose you are not the biggest person on the block, but you
have thousands of years to become one. What do you do? If you are
an animal, the most straightforward approach is becoming physically
bigger, like the alpha male in a dog pack, with selection favoring
muscle and bone. But there is another way to double your biomass.
It’s not by creating a body but by creating an ally. If you can establish
cooperative agreements with some of your neighbors, you can double
your power even if you do not personally double your strength. You
can dominate the world. Trying to fight off a woolly mammoth?
Alone, and the fight might look like Bambi vs. Godzilla. Two or three

52. BRAIN RULES
44
of you, however, coordinating your behaviors and establishing the
concept of “teamwork,” and you present a formidable challenge. You
can figure out how to compel the mammoth to tumble over a cliff,
for one. There is ample evidence that this is exactly what we did.
This changes the rules of the game. We learned to cooperate,
which means creating a shared goal that takes into account your
allies’ interests as well as your own. Of course, in order to understand
your allies’ interests, you must be able to understand others’
motivations, including their reward and punishment systems. You
need to know where their “itch” is.
Understanding how parenting and group behavior allowed us to
dominate our world may be as simple as understanding a few ideas
behind the following sentence: The husband died, and then the wife
died. There is nothing particularly interesting about that sentence,
but watch what happens when I add two small words at the end:
The husband died, and then the wife died of grief. All of a sudden we
have a view, however brief, into the psychological interior of the wife.
We have an impression of her mental state, perhaps even knowledge
about her relationship with her husband.
These inferences are the signature characteristic of something
called Theory of Mind. We activate it all the time. We try to see our
entire world in terms of motivations, ascribing motivations to our
pets and even to inanimate objects. (I once knew a guy who treated
his 25-foot sailboat like a second wife. Even bought her gifts!) The
skill is useful for selecting a mate, for navigating the day-to-day
issues surrounding living together, for parenting. Theory of Mind is
something humans have like no other creature. It is as close to mind
reading as we are likely to get.
This ability to peer inside somebody’s mental life and make
predictions takes a tremendous amount of intelligence and, not
surprisingly, brain activity. Knowing where to find fruit in the jungle
is cognitive child’s play compared with predicting and manipulating
other people within a group setting. Many researchers believe a direct

53. 2. SURVIVAL
45
line exists between the acquisition of this skill and our intellectual
dominance of the planet.
When we try to predict another person’s mental state, we have
physically very little to go on. Signs do not appear above a person’s
head, flashing in bold letters his or her motivations. We are forced
to detect characteristics that are not physically obvious at all. This
talent is so automatic, we hardly know when we do it. We began
doing it in every domain. Remember the line that we can transform
into a “1” and an “i”? Now you have dual representation: the line and
the thing the line represents. That means you have Judy DeLoache,
and that means you have us. Our intellectual prowess, from language
to mathematics to art, may have come from the powerful need to
predict our neighbor’s psychological interiors.
feeling it
It follows from these ideas that our ability to learn has deep
roots in relationships. If so, our learning performance may be deeply
affected by the emotional environment in which the learning takes
place. There is surprising empirical data to support this. The quality
of education may in part depend on the relationship between student
and teacher. Business success may in part depend on the relationship
between employee and boss.
I remember a story by a flight instructor I knew well. He told me
about the best student he ever had, and a powerful lesson he learned
about what it meant to teach her. The student excelled in ground
school. She aced the simulations, aced her courses. In the skies, she
showed surprisingly natural skill, quickly improvising even in rapidly
changing weather conditions. One day in the air, the instructor saw
her doing something naïve. He was having a bad day and he yelled
at her. He pushed her hands away from the airplane’s equivalent of a
steering wheel. He pointed angrily at an instrument. Dumbfounded,
the student tried to correct herself, but in the stress of the moment,
she made more errors, said she couldn’t think, and then buried her

54. BRAIN RULES
46
head in her hands and started to cry. The teacher took control of
the aircraft and landed it. For a long time, the student would not get
back into the same cockpit. The incident hurt not only the teacher’s
professional relationship with the student but the student’s ability to
learn. It also crushed the instructor. If he had been able to predict
how the student would react to his threatening behavior, he never
would have acted that way.
If someone does not feel safe with a teacher or boss, he or she
may not be able to perform as well. If a student feels misunderstood
because the teacher cannot connect with the way the student learns,
the student may become isolated. This lies at the heart of the flight
student’s failure. As we’ll see in the Stress chapter, certain types
of learning wither in the face of traumatic stress. As we’ll see in
the Attention chapter, if a teacher can’t hold a student’s interest,
knowledge will not be richly encoded in the brain’s database. As we
see in this chapter, relationships matter when attempting to teach
human beings. Here we are talking about the highly intellectual
venture of flying an aircraft. But its success is fully dependent upon
feelings.
It’s remarkable that all of this came from an unremarkable change
in the weather. But a clear understanding of this affords us our first
real insight into how humans acquire knowledge: We learned to
improvise off a database, with a growing ability to think symbolically
about our world. We needed both abilities to survive on the savannah.
We still do, even if we have exchanged it for classrooms and cubicles.

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Summary
Rule #2
The human brain evolved, too.
We don’t have one brain in our heads; we have three.
•
•
We started with a “lizard brain” to keep us breathing,
then added a brain like a cat’s, and then topped those with
the thin layer of Jell-O known as the cortex—the third,
and powerful,“human” brain.
We took over the Earth by adapting to change itself,
•
•
after we were forced from the trees to the savannah
when climate swings disrupted our food supply.
Going from four legs to two to walk on the savannah
•
•
freed up energy to develop a complex brain.
Symbolic reasoning is a uniquely human talent. It may
•
•
have arisen from our need to understand one another’s
intentions and motivations, allowing us to coordinate
within a group.
Get more at www.brainrules.net

57. wiring
Rule #3
Every brain is wired differently.

59. michael jordan’s athletic failures are puzzling, don’t
you think?
In 1994, one of the best basketball players in the
world—ESPN’s greatest athlete of the 20th
century—
decided to quit the game and take up baseball instead. Jordan failed
miserably, hitting .202 in his only full season, the lowest of any
regular player in the league that year. He simultaneously committed
11 errors in the outfield, also the league’s worst. Jordan’s performance
was so poor, he couldn’t even qualify for a triple-A farm team. Though
it seems preposterous that anyone with his physical ability would fail
at any athletic activity he put his mind to, the fact that Jordan did not
even make the minor leagues is palpable proof that you can.
His failure was that much more embarrassing because another
athletic legend, Ken Griffey Jr., was burning up the baseball diamond
that same year. Griffey was excelling at all of the skills Jordan seemed
to lack—and doing so in the majors, thank you. Griffey, then playing
for the red-hot Seattle Mariners, maintained this excellence for most

60. BRAIN RULES
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of the decade, batting .300 for seven years in the 1990s and at the
same time slugging out 422 home runs. He is, at this printing, sixth
on the all-time home-runs list.
Like Jordan, Griffey Jr. played in the outfield but, unlike Jordan,
he was known for catches so spectacular he seemed to float in the
air. Float in the air? Wasn’t that the space Jordan was accustomed to
inhabiting? But the sacred atmosphere of the baseball park refused to
budge for Jordan, and he eventually went back to what his brains and
muscles did better than anyone else’s, creating a legendary sequel to
a previously stunning basketball career.
What was going on in the bodies of these two athletes? What is
it about their brains’ ability to communicate with their muscles and
skeletons that made their talents so specialized? It has to do with
how their brains were wired. To understand what that means, we
will watch what happens in the brain as it is learning, discuss the
enormous role of experience in brain development—including how
identical twins having an identical experience will not emerge with
identical brains—and discover that we each have a Jennifer Aniston
neuron. I am not kidding.
fried eggs and blueberries
You have heard since grade school that living things are made
of cells, and for the most part, that’s true. There isn’t much that
complex biological creatures can do that doesn’t involve cells. You
may have little gratitude for this generous contribution to your
existence, but your cells make up for the indifference by ensuring
that you can’t control them. For the most part, they purr and hum
behind the scenes, content to supervise virtually everything you will
ever experience, much of which lies outside your awareness. Some
cells are so unassuming, they find their normal function only after
they can’t function. The surface of your skin, for example—all 9
pounds of it—literally is deceased. This allows the rest of your cells
to support your daily life free of wind, rain, and spilled nacho cheese

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at a basketball game. It is accurate to say that nearly every inch of
your outer physical presentation to the world is dead.
The biological structures of the cells that are alive are fairly easy
to understand. Most look just like fried eggs. The white of the egg
we call the cytoplasm; the center yolk is the nucleus. The nucleus
contains that master blueprint molecule and newly christened patron
saint of wrongfully convicted criminals: DNA. DNA possesses genes,
small snippets of biological instructions, that guide everything from
how tall you become to how you respond to stress. A lot of genetic
material fits inside that yolk-like nucleus. Nearly 6 feet of the stuff
are crammed into a space that is measured in microns. A micron is
1/25,000th
of an inch, which means putting DNA into your nucleus
is like taking 30 miles of fishing line and stuffing it into a blueberry.
The nucleus is a crowded place.
One of the most unexpected findings of recent years is that this
DNA, or deoxyribonucleic acid, is not randomly jammed into the
nucleus, as one might stuff cotton into a teddy bear. Rather, DNA is
folded into the nucleus in a complex and tightly regulated manner.
The reason for this molecular origami: cellular career options. Fold
the DNA one way and the cell will become a contributing member of
your liver. Fold it another way and the cell will become part of your
busy bloodstream. Fold it a third way and you get a nerve cell—and
the ability to read this sentence.
So what does one of those nerve cells look like? Take that fried
egg and smash it with your foot, splattering it across the floor. The
resulting mess may look like a many-pointed star. Now take one
tip of that star and stretch it out. Way out. Using your thumb, now
squish the farthest region of the point you just stretched. This creates
a smaller version of that multipronged shape. Two smashed stars
separated by a long, thin line. There’s your typical nerve. Nerve cells
come in many sizes and shapes, but most have this basic framework.
The foot-stomped fried-egg splatter is called the nerve’s cell body. The
many points on the resulting star are called dendrites. The region

62. BRAIN RULES
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you stretched out is called an axon, and the smaller, thumb-induced
starburst at the farther end of the axon is called the axon terminal.
These cells help to mediate something as sophisticated as human
thought. To understand how, we must journey into the Lilliputian
world of the neuron, and to do that, I would like to borrow from
a movie I saw as a child. It was called Fantastic Voyage, written by
Harry Kleiner and popularized afterward in a book by legendary
science-fiction writer Isaac Asimov. Using a premise best described as
Honey, I Shrunk the Submarine, the film follows a group of researchers
exploring the internal workings of the human body—in a submersible
reduced to microscopic size. We are going to enter such a submarine,
which will allow us to roam around the insides of a typical nerve cell
and the watery world in which it is anchored. Our initial port of call
is to a neuron that resides in the hippocampus.
When we arrive at this hippocampal neuron, it looks as if
we’ve landed in an ancient, underwater forest. Somehow it has
become electrified, which means we are going to have to be careful.
Everywhere there are submerged jumbles of branches, limbs, and
large, trunk-like objects. And everywhere sparks of electric current
run up and down those trunks. Occasionally, large clouds of tiny
chemicals erupt from one end of the tree trunks, after the electricity
has convulsed through them.
These are not trees. These are neurons, with some odd structural
distinctions. Hovering close to one of them, for example, we realize
that the “bark” feels surprisingly like grease. That’s because it is
grease. In the balmy interior of the human body, the exterior of the
neuron, the phospholipid bilayer, is the consistency of Mazola oil.
It’s the interior structures that give a neuron its shape, much as the
human skeleton gives the body its shape. When we plunge into the
interior of the cell, one of the first things we will see is this skeleton.
So let’s plunge.
It’s instantly, insufferably overcrowded, even hostile, in here.
Everywhere we have to navigate through a dangerous scaffolding

63. 3. WIRING
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of spiky, coral-like protein formations: the neural skeleton. Though
these dense formations give the neuron its three-dimensional shape,
many of the skeletal parts are in constant motion—which means we
have to do a lot of dodging. Millions of molecules still slam against
our ship, however, and every few seconds we are jolted by electrical
discharges. We don’t want to stay long.
swimming laps
We escape from one end of the neuron. Instead of perilously
winding through sharp thickets of proteins, we now find ourselves
free-floating in a calm, seemingly bottomless watery canyon. In the
distance, we can see another neuron looming ahead.
We are in the space between the two neurons, called a synaptic
cleft, and the first thing we notice is that we are not alone. We
appear to be swimming with large schools of tiny molecules. They
are streaming out of the neuron we just visited and thrashing
helter-skelter toward the one we are facing. In a few seconds, they
reverse themselves, swimming back to the neuron we just left. It
instantly gobbles them up. These schools of molecules are called
neurotransmitters, and they come in a variety of molecular species.
They function like tiny couriers, and neurons use these molecules
to communicate information across the canyon (or, more properly,
the synaptic cleft). The cell that lets them escape is called the pre-
synaptic neuron, and the cell that receives them is called the post-
synaptic neuron.
Neurons release these chemicals into the synapse usually in
response to being electrically stimulated. The neuron that receives
them can react negatively or positively when it encounters these
chemicals. Working something like a cellular temper tantrum, the
neuron can turn itself off to the rest of the neuroelectric world (a
process termed inhibition). Or, the neuron can become electrically
stimulated. That allows a signal to be transferred from the pre-
synaptic neuron to the post: “I got stimulated and I am passing on

64. BRAIN RULES
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the good news to you.” Then the neurotransmitters return to the cell
of origin, a process appropriately termed re-uptake. When that cell
gobbles them up, the system is reset and ready for another signal.
As we look 360 degrees around our synaptic environment,
we notice that the neural forest, large and seemingly distant, is
surprisingly complicated. Take the two neurons between which we
are floating. We are between just two connection points. If you can
imagine two trees being uprooted by giant hands, turned 90 degrees
so the roots face each other, and then jammed together, you can
visualize the real world of two neurons interacting with each other
in the brain. And that’s just the simplest case. Usually, thousands of
neurons are jammed up against one another, all occupying a single
small parcel of neural real estate. The branches form connections
to one another in a nearly incomprehensible mass of branching
confusion. Ten thousand points of connection is typical, and each
connection is separated by a synapse, those watery canyons in which
we are now floating.
Gazing at this underwater hippocampal forest, we notice several
disturbing developments. Like snakes swaying to the rhythm of
some chemical flute, some of these branches appear to be moving.
Occasionally, the end of one neuron swells up, greatly increasing
in diameter. The terminal ends of other neurons split down the
middle like a forked tongue, creating two connections where there
was only one. Electricity crackles through these moving neurons
at a blinding 250 miles per hour, some quite near us, with clouds
of neurotransmitters filling the spaces between the trunks as the
electric current passes by.
What we should do now is take off our shoes and bow low in
the submarine, for we are on Holy Neural Ground. What we are
observing is the process of the human brain learning.
extreme makeover
Eric Kandel is the scientist mostly responsible for figuring out

65. 3. WIRING
57
the cellular basis of this process. For it, he shared the Nobel Prize
in 2000, and his most important discoveries would have made
inventor Alfred Nobel proud. Kandel showed that when people learn
something, the wiring in their brains changes. He demonstrated that
acquiring even simple pieces of information involves the physical
alteration of the structure of the neurons participating in the process.
Taken broadly, these physical changes result in the functional
organization and reorganization of the brain. This is astonishing. The
brain is constantly learning things, so the brain is constantly rewiring
itself.
Kandel first discovered this fact not by looking at humans but
by looking at sea slugs. He soon found, somewhat insultingly, that
human nerves learn things in the same way slug nerves learn things.
And so do lots of animals in between slugs and humans. The Nobel
Prize was awarded in part because his careful work described the
thought processes of virtually every creature with the means to
think.
We saw these physical changes while our submarine was
puttering around the synaptic space between two neurons. As
neurons learn, they swell, sway, and split. They break connections
in one spot, glide over to a nearby region, and form connections
with their new neighbors. Many stay put, simply strengthening their
electrical connections with each other, increasing the efficiency of
information transfer. You can get a headache just thinking about the
fact that deep inside your brain, at this very moment, bits of neurons
are moving around like reptiles, slithering to new spots, getting fat
at one end or creating split ends. All so that you can remember a few
things about Eric Kandel and his sea slugs.
But before Kandel, in the 18th
century, the Italian scientist
Vincenzo Malacarne did a surprisingly modern series of biological
experiments. He trained a group of birds to do complex tricks, killed
them all, and dissected their brains. He found that his trained birds
had more extensive folding patterns in specific regions of their

66. BRAIN RULES
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brains than his untrained birds. Fifty years later, Charles Darwin
noted similar differences between the brains of wild animals and
their domestic counterparts. The brains in wild animals were 15 to
30 percent larger than those of their tame, domestic counterparts.
It appeared that the cold, hard world forced the wild animals into a
constant learning mode. Those experiences wired their heads much
differently.
It is the same with humans. This can be observed in places
ranging from New Orleans’s Zydeco beer halls to the staid palaces
of the New York Philharmonic. Both are the natural habitat of
violin players, and violin players have really strange brains when
compared with non-violin players. The neural regions that control
their left hands, where complex, fine motor movement is required
on the strings, look as if they’ve been gorging on a high-fat diet.
These regions are enlarged, swollen and crisscrossed with complex
associations. By contrast, the areas controlling the right hand, which
draws the bow, looks positively anorexic, with much less complexity.
The brain acts like a muscle: The more activity you do, the
larger and more complex it can become. Whether that leads to more
intelligence is another issue, but one fact is indisputable: What you
do in life physically changes what your brain looks like. You can
wire and rewire yourself with the simple choice of which musical
instrument—or professional sport—you play.
some assembly required
How does this fantastic biology work? Infants provide a front-row
seat to one of the most remarkable construction projects on Earth.
Every newly born brain should come with a sticker saying “some
assembly required.” The human brain, only partially constructed
at birth, won’t be fully assembled for years to come. The biggest
construction programs aren’t finished until you are in your early 20s,
with fine-tuning well into your mid-40s.
When babies are born, their brains have about the same number

67. 3. WIRING
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of connections as adults have. That doesn’t last long. By the time
children are 3 years old, the connections in specific regions of their
brains have doubled or even tripled. (This has given rise to the
popular belief that infant brain development is the critical key to
intellectual success in life. That’s not true, but that’s another story.)
This doubling and tripling doesn’t last long, either. The brain soon
takes thousands of tiny pruning shears and trims back a lot of this
hard work. By the time children are 8 or so, they’re back to their
adult numbers. And if kids never went through puberty, that would
be the end of the story. In fact, it is only the middle of the story.
At puberty, the whole thing starts over again. Quite different
regions in the brain begin developing. Once again, you see frenetic
neural outgrowth and furious pruning back. It isn’t until parents
begin thinking about college financial aid for their high schoolers
that their brains begin to settle down to their adult forms (sort of).
It’s like a double-humped camel. From a connectivity point of view,
there is a great deal of activity in the terrible twos and then, during
the terrible teens, a great deal more.
Though that might seem like cellular soldiers obeying growth
commands in lockstep formation, nothing approaching military
precision is observed in the messy world of brain development. And
it is at this imprecise point that brain development meets Brain Rule.
Even a cursory inspection of the data reveals remarkable variation
in growth patterns from one person to the next. Whether examining
toddlers or teenagers, different regions in different children develop
at different rates. There is a remarkable degree of diversity in the
specific areas that grow and prune, and with what enthusiasm they
do so.
I’m reminded of this whenever I see the class pictures that
captured my wife’s journey through the American elementary-school
system. My wife went to school with virtually the same people for
her entire K–12 experience (and actually remained friends with most
of them). Though the teachers’ dated hairstyles are the subject of

68. BRAIN RULES
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much laughter for us, I often focus on what the kids looked like back
then. I always shake my head in disbelief.
In the first picture, the kids are all in grade one. They’re about
the same age, but they don’t look it. Some kids are short. Some are
tall. Some look like mature little athletes. Some look as if they just
got out of diapers. The girls almost always appear older than the boys.
It’s even worse in the junior-high pictures of this same class. Some
of the boys look as if they haven’t developed much since third grade.
Others are clearly beginning to sprout whiskers. Some of the girls,
flat chested and uncurved, look a lot like boys. Some seem developed
enough to make babies.
Why do I bring this up? If we had X-ray eyes capable of
penetrating their little skulls, we would find that the brains of these
kids are just as unevenly developed as their bodies.
the jennifer aniston neuron
We are born into this world carrying a number of preset circuits.
These control basic housekeeping functions like breathing, heartbeat,
your ability to know where your foot is even if you can’t see it, and
so on. Researchers call this “experience independent” wiring. The
brain also leaves parts of its neural construction project unfinished
at birth, waiting for external experience to direct it. This “experience
expectant” wiring is related to areas such as visual acuity and perhaps
language acquisition. And, finally, we have “experience dependent”
wiring. It may best be explained with a story about Jennifer Aniston.
You might want to skip the next paragraph if you are squeamish.
Ready? A man is lying in surgery with his brain partially exposed
to the air. He is conscious. The reason he is not crying out in agony
is that the brain has no pain neurons. He can’t feel the needle-sharp
electrodes piercing his nerve cells. The man is about to have some
of his neural tissue removed—resected, in surgical parlance—
because of intractable, life-threatening epilepsy. Suddenly, one of the
surgeons whips out a photo of Jennifer Aniston and shows it to the